y'=3[cos(1/x)]^2*[cos(1/x)]'..=3[cos(1/x)]^2*[-sin(1/x)]*(1/x)'..=3[cos(1/x)]^2*[-sin(1/x)]*(-1/x^2)..=3[cos(1/x)]^2[sin(1/x)]/x^2所以dy=3[cos(1/x)]^2[sin(1/x)]/x^2 dx